In known conveying systems of the self-propelled platform car type, a 3-wheeled SPPC is provided at its front center with a motor-driven wheel, a steering guide rail laid below the rolling route of that driven wheel, in a pit below the floor. A steering trolley is engaged with the guide rail and a wheel shaft bearing capable of steering movement, which journals the shaft of the driven wheel is connected through a coupling member vertically ranging through a slit formed in the floor, at the center of the rolling route of the driven wheel.
In such a conventional conveying system, the steering guide rail and the steering trolley to be engaged by the guide rail are located below the driven wheel at the front center of the SPPC or below the central position of the SPPC. Therefore, even if floor plates placed along the slit are removed, such as in the case of repairing the trolley, work access is hampered by the SPPC that is located above the steering trolley.
Even if the driven wheel at the front center were to be steered by mechanical constraint along the steering guide rail, but since no means were available for subjecting the rear pair of wheels on the platform car to a constrained, guided steeringly following the steering guide rail, the resistance to running was relatively large, when this SPPC ran long a curved route. Therefore, this required a considerably high driving force.